A spark plug used for an automotive internal combustion engine employs a center electrode and a ground electrode for generating the spark there-between.
The rich air fuel mixture is supplied to the automotive internal combustional engine, in order to improve the driving condition under the low temperature atmosphere, so that carbon which is not a conductive material may deposit on a surface of a insulator which insulates the center electrode from the ground electrode. As to the present inventors' experiment, it is observed that the carbon is deposited on the insulator during the beginning stage of the operation of the engine, namely during the transferring stage while the automotive is transferred from the automotive manufactory to the user. The carbon deposited on the insulator reduces the insulating effect so that the carbon reduces the life length of the spark plug.
In order to prevent the disadvantage caused by the carbon, the conventional type of spark plug(Japan Patent 56-51476) has employed the center electrode the top portion of which is narrower than the other parts so that a ring shaped space is formed between the top portion of the center electrode and the insulator, and the top end of the center electrode is withdrawn from the top surface of the insulator The conventional type of spark plug(Japan patent 56-51476) has employed the ground electrode, the side surface of which is provided close to the insulator in such a manner that a gap between the side surface of the ground electrode and the top end of the insulator is narrower than a gap between a top end portion of the center electrode and the side surface of the ground electrode. A spark is generated at the first gap between the center electrode and the ground electrode when the carbon is not deposited on the top surface of the insulator. The spark then generates at the second gap between the insulator and the ground electrode when the carbon is deposited within the ring shaped space in order to burn out the carbon deposited within the ring shaped space.
Another type of conventional spark plug(Japan patent 58-40831) has employed the center electrode, the top portion of the center electrode being narrower than the remaining portion so that the ring shaped space is formed between the outer surface of the top portion of the center electrode and the inner surface of the insulator, the top end of which is extruded from the top surface of the insulator. The ground electrode of the conventional type of spark plug(Japan patent 58-40831) faces toward the side surface of the top portion of the center electrode which is extruded from the insulator in such a manner that a first gap is formed between the top end of the ground electrode and the side surface of the center electrode. A second gap which is smaller than the first gap is formed between the top surface of the insulator and the side surface of the ground electrode of the conventional spark plug. The spark is generated at the first gap while the carbon is not deposited on the top surface of the insulator, and the spark is generated at the second gap when the carbon is deposited within the ring shaped space including the top portion of the insulator. The spark generated at the second gap burns out the carbon deposited within the ring shaped space.
These conventional types of spark plugs, however, have disadvantages described hereinafter. Since the top end of the center electrode of the former spark plug, (Japan patent 56-51476), is withdrawn into the inner portion of the insulator, the spark generated at the first gap should contact with the inner surface of the insulator while the core of the flare grows, so that the growth of the core of the .flare is hindered by the inner surface of the insulator. Accordingly, the former type of the conventional spark plug cannot ignite effectively. Furthermore, since the second gap is narrower than the first gap of the former type of conventional spark plug(Japan patent 56-51476), the core of the flare cannot grow at the second gap even when the spark is generated at the second gap under the condition that the carbon is deposited within the ring shaped space. The conventional spark plug, therefore, cannot ignite effectively.
Since the first gap of the latter type of the conventional spark plug (Japan patent 58-40831) is formed at the side surface of the top portion of the center electrode which is extruded from the insulator, the core of the flare at the first gap can grow more smoothly than that of the former type of the conventional spark plug. However, since the second gap of the latter type of the conventional spark plug is positioned behind of the first gap, the core of the flare generated at the second gap is hard to be contacted with the air-fuel mixture. Furthermore since the second gap is narrower than the first gap, the core of the flare generated at the second gap cannot grow widely so that the core of the flare generated at the second gap cannot ignite the air-fuel mixture effectively.
Accordingly, the disadvantage that the growth of the core of the flare generated at the first gap is hindered by the contact with the inner surface of the insulator such as caused in the former type of the conventional spark plug is solved by extruding the top end of the center electrode from the top end of the insulator such as described in the latter type of spark plug.
However, since the second gap of both types of the conventional spark plug is narrower than the first gap, the disadvantage that the second gap at which the spark is generated when the carbon is deposited within the ring shaped space cannot attain the effective igniting.